Signal transmission system



Oct. 22, 193.5. ,L S'JDQBAY JR I 2,018,489

SIGNAL TRANSMISSION SYSTEM Filed Aug. 12, 1952 f ir Patented Oct. 22,1935 UNITED STATES ATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application August 12,

20 Claims.

v'Ihis invention relates to signal transmission systems and particularlyto the control of the volume range of the signals on transmissionsystems.

One object of the invention is to provide a signal system that shall inan improved manner compress the signals a fraction of the originalvolume range before transmission over a line and expand the signals atthe receiving end of the transmission line to the original volume range.

lAnother object of the invention is to provide a signal system thatshall compress the signals a fraction of the original volume range atthe transmitting end of a line under the control of a backward actingcircuit and expand the signals to the original volume range at thereceiving end of the line.

A further object of the invention is to provide a signal system thatshall compress the volume range of signals a fraction of the originalvolume range before transmission over a line by threeelement spacedischarge devices connected across the line under the control of abackward acting circuit and that shall expand the volume range of .the`signals to the original range at the receiving end of the line bythree-element space discharge devices connected effectively in serieswith the line under the control of a forward acting circuit.

The signals received for transmission over a line in many instances havea volume range larger than can be ef-Iiciently transmitted. Signalsystems are limited as to the volume range which can be satisfactorilytransmitted. Satisfactory transmission of signals has an upper limit toavoid overloading of line apparatus such as repeaters and interferencewith other circuits and has a lower limit by reason of interference fromline noises.

The present invention discloses a system wherein the volume range of thetransmitted signals is held within the upper and lower limits ofsatisfactory transmission by changing the shape of the signal Waves-atthe transmitting end of the line and restoring the signal waves to theoriginal shape at the receiving end of the line. The invention is animprovement over the system disclosed in the S. Doba, Jr. Patent No.1,931,063, dated October 17, 1933 and the patent to S. Doba No.1,854,828 dated April 19, 1932.

In the present invention, the compressor for reducing the volume rangeof the signals a fraction of the original volume range comprises twothree-element space discharge devices connected across the transmissionline and a backward act- 1932, Serial N0. 628,505

ing circuit for controlling the grid bias on the devices. The backwardacting control circuit includes a linear operating rectifier. Thecharacteristic of the rectier vand the characteristics of the two spacedischarge devices have a deiinite` volume range. e

The expander at the receiving end of the line for expanding the volumerange of the signals to the original volume range is very similar incongo struction and operation to the expander disclosed in the abovementioned patent to S. Doba, Jr., No. 1,931,063. The impedance meansemployed by the expander comprises two three-element space dischargetubes which are effectively connected in $5` series with the line. 'I'hegrid bias on the space discharge tubes is controlled by a forward actingcircuit having a linear acting rectifier. The characteristics of thespace discharge tubes and the rectier are adjusted in the mannerdisclosed in 30 the S. Doba, Jr. Patent No. 1,931,063.

In a compressor of the type employed in this invention a fixed relationis maintained between the characteristics of the space discharge devicescomprising the impedance element and the charu acteristic of therectifier in the backward acting control circuit for the impedanceelement. The relation between the characteristics of the impedanceelement and the rectier is set forth be1ow. 0

If the current voltage characteristic of the lmpedance element is I KE'then a 1. 25 R`DE the impedance, is

50 d V1 R0* nKE-1 E is the potential impressed on the grids of theimpedance element and K is a constant.

In a shunt type oi' variable repeater which is 55 10g E=20 logKa-l-'rLiVo (3) VCombining Equations (2) and (3) G:2o1ogK2- n-o2010g1f3+mv0;

" K4-(7L-1ML1V0 (4) Y employed in the compressor, the amplification isdirectly proportional to the impedance of the variable impedanceelement.

K1 K2 A nK n-l-En-l A is the voltage amplication, K1 and K2 areconstants and the gain in decibels is:

If E is obtained from a rectiiier circuit with a characteristic YE=K360TL1 where E is the output voltage of the rectiiier circuit and e0is the input voltage of the rectiiier circuit and the output Voltage ofthe variable repeater, Vthen 20 log E=2O log KS4-m20 logr eo (20 log en)`may be called the output volume Vo of the variable repeater, hence Theoutput volume of the Variable repeater is the input volume plus the gainor Where V1 is the input volume to the variable repeater in decibels.

lSubstituting the value of G Equation (4) in Equation (5) The constantKi'may be arbitrarily given a value of Zero in Equation (6) and If theoutput volume range of the variable repeater is to vary as a fraction Fof input Volume range,

Y The valueV of n may be varied by the means disclosed in the patent toS. Doba, Jr., No. 1,931,063 or by using more than one stage of im- Ypedance elements in tandem.` The Value of m may beV controlled by meansof rooter devices or squarer devices as disclosed in the patent to G.Crisson No. 1,737,830 dated' December 3, 1929.

' YIvhe amount of rooting or squaring may be controlled by the meansdisclosed in the above mentionedS. Deba, Jr., Patent No. 1,931,063.

v In the present application of the invention, the

value of F is assumed to be 1/2 and the Variable impedance element to beadjusted so that n has a value of 2. The value 11,1 derived from Equaftion (8) is 1.

If it were desired to have a value for F of 1/3,Y when n is equal to 2,then Equation (8) gives a Value for n1 of 2.

In considering the characteristics of the impedance element andtherectifier circuit of the expander at the receiving end of the line itshould `be, noted the. rectier circuit is forward acting and the spacedischarge tubes of the impedance element are connected effectively inseries with the vline5 fstart the percussive consonants.

pressors and expanderinthe present invention amplification is inverselyproportional to the 1mpedance of the variable impedance element.

A=K17tKEn`1:KzEL-l (9) The gain in decibels is:

G=20 log A=20 log K2+(n-1)20 log E (10) If E is obtained from a rectiercircuit with a. characteristic, i

E=K3ein1 Where E is the output voltage of theV rectiiier circuit and ei1s the input voltage of the rectifier circuit and also the input voltageto the Variable Y impedance element, then 20 log E=20 log E34-m20 logVei The value (20 log ei) may be called the input volume V1 to thevariable impedance element. Substituting V1 for 20 log e1, y f

20 log E=20 log K3-l-V7L1Vi (11) Substituting for 2O log Ev Equation(l1) in Equation (1 0) v The output volume from the variable repeater ofthe expander is the input volume plus the gain.

Vo=VilGY Y (13) The output volume V0 is given in decibels.Y Substitutingthe value of G Equation (12) in Equation (13) The constant K4 may bearbitrarily given a value of zero in Equation (14) if the output volumefrom the variable re peater of the expander varies inversely accordingto a fraction F The values oi n and n1 may be varied in the same manneras the values of n and ln1 ink the compressor. The expander in operationis complementary to the operatien of the compressor.

In a compressor having a backward-acting control circuit as employed inthe present invention there is no over-accentuation of the initial partof the syllables which start with percussive consonants.V Inacompressor' having a; forward- Vactingccntrol circuit there is atendency for the compressor and expander in tandem to overaccentuate theinitial part of the syllables that The comhave no such tendency byreasonjo-f the compressor having a backward-acting control circuit andthe expander having a forward-acting control circuit. Signals havingidentically the same variations in volume range are applied'to the twocontrol circuits. The term signal when used in the specification andclaim is intended to refer not only to the communication of intelligencebut also to the transfer of music and other programs. f

In the accompanying drawing,

Fig. l is a diagrammatic view of a compressor-- expander circuitconstructed in accordance with the invention;

Fig. 2 is a diagrammatic View of a modified circuit for controlling thecompressor.

Referring to 1Fig. l of the drawing a compressor I and an expander 2 areshown connected to a transmission line having input conductors 3 and 4and output conductors 5 and 6. 'I'he `compressor i comprises a variablerepeater 1 under the Vcontrol of a backward-acting rectifier circuit 8.IIhe expander 2 comprises a variable re- Y peater 5 under the control ofa forward-acting rectifier circuit I9.

The variable repeater 1 in the compressor` I comprises an impedanceelement I I and two ampliers I2 and I3. The amplifiers I2 and I3 arepreferably of the pure electron discharge type. The impedance element IIcomprises two threeelement space discharge devices I4 and I5 which aresymmetrically connected across the line circuit between resistanceelements I6, I1, I8 and I9. The resistance elements I1 and I9, serve toeffect a rough adjustment of the characteristics of the space dischargedevices I4 and I5 in the manner disclosed in the above mentioned patentto S. Doba, Jr., No. 1,931,063.

The device I4 comprises a grid 2l, a cathode 22 and a plate 23. Thedevice I5 comprises a grid 24, a cathode 25 and a plate 28. A battery 21is provided for supplying plate current to the devices I4 and I5 and abattery 28 is provided for supplying negative bias to the grids 2| and24 of the devices I4 and I5. A variable grid bias 'for the devices I4and I5 is supplied by the control or rectifier circuit 9. Resistanceelements 29 are provided in the grid-filament circuits of the devices I4and I5 to effect a fine adjustment of the characateristics of thedevices I4 and I5.

The impedance element is connected to the input conductors 3 and 4 bymeans of a transformer 38 and is connected to the amplifier I2 by meansof a transformer 3l. The amplifier I2 is connected to the amplifier I3by a transformer 32. The amplifier I3 is connected to a high-pass filter33 by means of a transformer 34. rPrefera ly, the filter 33 will passwaves only in the speech band to prevent the operation of the rectifiercircuit 8 by the transients due to the unbalance of the devicesM and I5or the unbalance of primary windings of the transformer 3l. Theamplifier I2 is provided with a grid biasing battery 35 and platebattery 35. The

lamplier I3 is provided with a grid biasing battery 31 and aplatebattery 38.

The rectifier circuit comprises a rectifier 48 of the space dischargetype and an amplifier 4I of the space discharge type. The input circuitof the amplifier 4! is connected by a transformer 42 to the line circuitbeyond the high-pass filter The output circuit of the amplifier 4I isconnected by a transformer 43 through the rectifier and a resistancecondenser network 44 to the 'grids 2l and of the space discharge devicesIta'nd I5. The resistance condenser network 44 lpreferably comprises acondenser 45 and a resistance element 45. rIhis rnetwork serves tosmooth out the ripples in the rectified current supplied to the grids oithe space discharge devices I4 and I5. A low-pass filter as shown in thepatent to S. Doba, Jr., No. 1,931,063 may also be employed.

The variable repeater 9 in the expander 2 comprises an impedance element41 and an amplifien 5 48. The impedance element 41 comprises two spacedischarge tubes 49 and 59 which are Aconnected effectively in serieswith the line circuit. In the compressor circuit it will be noted thetwo space discharge devices I4 and I5 are con-'10 ing a rough adjustmentof the characteristics of 20 the space discharge tubes 49 and 50. Acondenser 51 is provided as an alternating current bypass across theresistance elements 55 and 55. Resistance elements 58 are provided inthe gridfilament circuits of the space discharge tubes 49725 and foreffecting a fine adjustment of ythe characteristics of the tubes. Thetubes'49 and 50 are provided with a grid biasing battery 65 and a platebattery 6I Variable grid bias for the tubes 49 and 50 is provided by theforward-acting recti-A 30 fier or control circuit I0.

The rectifier circuit I0 comprises a rectifier 83 of the space dischargetype and an amplifier 64 preferably of the space discharge type. Theinput circuit of the amplifier 64 is connected to the linei'35 ahead ofthe transformer 5I by means of a transformer 95. The output circuit ofthe amplifier is connected by transformer 66 through theY rectifier 63and a condenser resistance network 61 to the grids of the spacedischarge tubes 49 and 5I).v The 40 resistance condenser network 81 isthe same type as the network 44 in the rectifier circuit 8 and comprisesa condenser 68 and the resistance element (i9. By making the networks atthe trans-v mitting and receiving ends of the line identical, 45

any delay in compressing action at the transmitting end of the line isbalanced by an equal delay at the receiving end of the line inexpanding. In

'this way over accentuation of percussive con-A sonants is avoided. Alow-pass filter may also be' 50 used here when one is used in thecompressor for the same reason.

In the compressor-expander circuit above described the compressor Icompresses the volume range of the signals received from the conductors3 and 4 by one-half. The characteristics of the impedance element I Iand the rectifier circuit 8 are adjusted together in o-rder to obtainthe correct compression. Inasmuchas the rectifier circuit isbackward-acting, itis unnecessary to"00 provide rooter devices of thetype disclosed in the above mentioned Crisson patent in order to obtainan output from the variable repeater 1 which has an output in decibelswhich is half the The expander input to the variable repeater. 2operates in the same manner as the expander disclosed in patent to S.Doba, Jr., No. 1,931,063. In Fig. 2 of the drawing is disclosed amodified rectifier or control circuit for the variable re-m.

peater of the compressor. The same circuit may To also be used for theexpander. Similar parts in the control circuit shown in Fig. 2 to thoseshown in Fig. l will be indicated by like reference characters. In thecontrol circuit shown in Fig, 21s

of the drawing a network comprising condensersv u V'aus Y drawing.

times. drawing the ccndensers i and il are charged up :10, 14|,inductance element 12 and rectifier 13 is substituted for the resistancecapacity network 44 shown in the control circuit 8 in Fig. 1 of the Theresistance capacity network 44 shown in Fig. 1 of the drawing isquick-acting so Vthat the control followsthe envelope of the speechWaves without causing any appreciable delay. However, in such a circuitthe gain does not remain constant at any time but is continuouslychanging. The network shown in Fig. 2 of the drawing has the advantageof the quick action of the network 44 shown in Fig. lof the drawing anddoes not have the disadvantage of the network 44 in that the gain ischanging at all In the network shown in Fig. 2 of the practicallysimultaneously upon the application of a signal to the input of therectier circuit due to the Yshort circulting of the inductance 'I2 bythe cathode-anode impedance of the rectifier tube 13. When the input tothe rectifier circuit is removed the flow of current through the networkis reversed and the impedance of rectifier tube 'i3 becomes infinite.The discharge of the condensers Hi and 'Il must then take place throughY element connected to said path, a rectifier circuit connected to saidpath beyond the connection of the impedance element to the path forcontrolling the impedance element, and means for effecting a definiterelation between the characteristics of the impedance element and therectifying circuit to obtain a variation in volume output on the pathbeyond the impedance element which is a fixed fraction of the variationin volume input on the path before the impedance element.

2. In combination, a signal line, impedance means comprisingthree-element spacer discharge devices connected to said line, meanscomprising a'control circuit having a rectifier circuit connected tosaid line beyond said impedance vmeans for controlling the impedancemeans, and means for effecting a definite relation between the operatingcharacteristics of said spaceV discharge devices and the operatingcharacteristic of said rectifier circuit to obtain a variable outputvolume on the line beyond said impedance means which is a fixed fractionof the range of the input signal volume expressed in decibels on theline before the impedanceV means.

' 31 InV combination,YV a signal line, impedance means comprisingthree-element space discharge devices connected across the line, meanscomprising a backward acting control circuit connected to the linebeyond said impedance means for controlling the bias on the grids ofsaid devices, and means for maintaining a fixedY relation between theoperating characteristic of the impedance-'means and the operatingcharacteristic of said control circuit to obtain a variable outputvolume on the line beyond said impedance means which is a fixed fractionof the range of the input signal Yvolume expressed in decibels on theline before the impedance means.

4. In combination, a signal line, impedanceV .means comprisingthree-element space discharge controlling the bias on the grids of saiddevices, 5

the operating characteristics of said space discharge devices and theoperating characteristic of said rectifier having a definite relation toobtain an output on the line beyond said impedance means which is halfthe range of signal k volume expressed in decibels on the line beforethe impedance means. l

5. In combination, a signal line, an impedance element connected acrosssaidy line, and means comprising a linear acting rectifier elementconnected to said Vline beyond the connection of the impedance elementto the line and havingV a definite relation between its operatingcharacteristic and the operating characteristic of said impedanceelement for controlling the impedance element to obtain an output on theline beyond said impedance element which is half the range of signalvolume expressed in n decibels :on the line before the impedanceelement.

6. In combination, a signal line, impedance means comprising twothree-element space discharge devices -connected across said line, aback- Ward-acting control circuit connected to said line beyond saidimpedance means, and having a rectifier for controlling the bias on thegrids of said devices and means for maintaining a fixed relation betweenthe characteristics of said space discharge devices and said rectifierto obtain a variation in volume output on the line beyond said impedancemeans which is av flxed fraction of the variation in volume input on theline before the impedance means.

7. In a signal system, a transmission line, two

space discharge devices connected across saidY line at the transmittingend thereof, means com-,w prising a backward-acting control circuitcon-`A` nected to said line beyond said discharge devices forcontrolling the devices to obtain an output on the line beyond saiddevices which is a fractional part of the range of signal volumeexpressed in decibels on the line before the devices, two spacedischarge tubes connected effectively in series with the line at thereceiving end thereof, Vand means comprising a for-k ward-acting controlcircuit connected to said.50 line before said space discharge tubes forcontrolling the tubes to obtain anV output on the line beyond the tubeshaving the same volume range as the volume inputv to saidV devices atthe transmitting end of the line.

8. In a signal system, a transmission line, threeelement space dischargedevices connected across said line at the transmitting end thereof forcontrolling the volume range of the transmitted signais, meanscomprising a backward-acting control circuit having a rectifierconnected to said line beyond said discharge devices for controlling thedevices, the operating characteristics of said devices and theVoperating characteristic of said control circuit having a definiterelation to ,65 obtain a variable volume output on the line beyond saiddevices which isa lxed fraction of the variations in the input signalvolume expressed in decibels on the line before the devices,threeelement space discharge tubes connected, eflectively in series withthe line at the receiving end thereof, and means comprising aforward-acting control circuit connected to said line before said tubesfor controlling the tubes to obtain an output onV the line beyond thetubes 7| mitting end thereof for reducing the range of the transmittedsignals, impedance means at the receiving end of the line for expandingthe Volume range of the transmitted signals, control means governedaccording to the volume range on the line beyond the impedance means atthe transmitting end of the line for controlling the impedance means atthe transmitting end of the line to obtain a fractional reduction in thevolume range of the transmitted signals, and oo nftrol means at thereceiving end of the line governed according to the same volume rangegoverning the control means at the transmitting end of the line forcontrolling the impedance means at the receiving end of the line toobtain an out- .'L-put beyond the impedance means at the receiving endof the line having the same volume range as the input to the impedancemeans at the transmitting end of the line.

10. In a signal system, a transmission lin --'space discharge devicesconnected across the line at the transmitting end thereof for reducingthe range of the transmitted signals, a backward-acting control circuitfor controlling said device to obtain an output on the line beyond saiddevices Which is a fractional part of the range of signal volumeexpressed in decibels on the line before the devices, space dischargetubes connected effectively in series with the line at tlie receivingend thereof for expanding the range of the transmitted signals, andcontrol means at the receiving end oi' the line governed according tothe same volume range supplied to said backward-acting circuit forcontrolling said discharge tubes to obtain an output beyond the tubeshaving the same volume range as the input to the devices at thetransmitting end of the line.

ll. In combination, a signal line, space discharge impedance means insaid line for controlling the impedance on said line, means comprising arectifier circuit connected to said line beyond the impedance means andhaving an output proportional to the output on the line beyond theimpedance means for controlling the impedance means, and means foreiecting a definite relation between the characteristics of theimpedance means and the rectier circuit to obtain a variation in volumeoutput on thev linel beyond the impedance. means which is a xed fractionof the variation in Volume on the line before the impedance means.

12. In combination, a signal line, space discharge impedance means insaid line for controlling the impedance on the line, a control circuitconnected to the line beyond said impedance means and comprising arectifier and a filter for controlling said impedance means, and meansfor effecting a definite relation between the characteristics of theimpedance means and the control circuit to obtain variation in volumeoutput on the line beyond the impedance means which is a fixed fractionof the variation in volunie input on the line before the impedancemeans.

13. In combination, a signal line, space discharge impedance means insaid line for controlling the impedance on the line, means comprising acontrol circuit having a rectifier and a iilter and connected to theline beyond said impedance for controlling the impedance means,

the output from said rectiiier being proportional to the output on theline beyond the impedance means, and means for effecting a definiterelation between the operating characteristics of said impedance meansand the operating char- 5 acteristics of said control circuit to obtaina variable output volume on the line beyond said impedance means whichis a fixed fraction of the range of the input signal volume expressed indecibels on the line before the impedance means.

lli. In combination, a signal line, impedance means in said line, meanscomprising a rectifier circuit connected to said line, beyond theimpedance means for controlling the impedance means, 15 a network insaid rectifier circuit comprising a capacity and an inductance shuntedby a rectifier for varying the time of charging the capacity as comparedwith the time of discharging the capacity, and means lfor effecting adefinite relation between the characteristics of the impedance elementand the rectifier circuit to obtain a variation in volume output on theline beyond the impedance means Which is a fixed fraction of thevariation in volume input on the line 25 before the impedance means.

l5. Inl combination, a control circuit, an inductance element connectedin series with said circuit, a capacity element connected across saidcircuit beyond said inductance element, and a 30 one-Way rectiiierconnected across said inductance element to insure charging anddischarging of the capacity element at different rates.

i6. In combination, a control circuit, means for applying direct currentimpulses to said cir- 35 cuit, an inductance element connected in serieswith said circuit, capacity means connected across said circuit adjacentto said inductance element, and a rectiiier connected across saidinductance element for transmitting said direct 40 current impulses inone direction to insure charging and discharging of the capacity meansat different rates.

17. In combination, a signal line, impedance means connected to thesignal line, control means 45 governed according to the volume range onthe lsignal line beyond said impedance means for controlling saidimpedance means, means for relating the characteristics of saidimpedance means and the control means to obtain a signal 50 volume rangebeyond said impedance means Which has a fixed ratio with respect to thesignal volume range before said impedance means, an expander lineliaving signals supplied thereto that have the same volume range as theoutput 55 beyond the rst mentioned impedance means, a second impedancemeans connected to the eX- pandor line, a second control means governedby the same wave shape as said first mentioned control means forcontrolling said second impedance means, and means for relating thecharacteristics of said second impedance means and the second controlcircuit to obtain an output beyond the second impedance means having thesame volume range as the input to the first mentioned impedance means.

18. The method of controlling the volume range of signals on a signalline having an impedance connected thereto and a control circuittherefor and an expandor line having a second impedance conected theretoand a second control circuit therefor which consists in governing saidsignalline control circuit according to the signal output beyond saidsignal line impedance, in controlling the signal-line impedance by thesignal-line control circuit, in relating the characteristic of thesignal-line impedance to the characteristic o'I the signal-line controlcircuit to obtain a signal volume beyond said signal-line impedanceWhich has a xed ratio to the signal volume range before said signal-lineimpedance, in governing the eXpandor-line control circuit by the samewave shape as controls said signal-line control circuit, in supplyingsaid expandor with signals corre spendingtothe output from thesignal-line impedance, in controlling the eXpandor-line impedance by theeXpandor-line control circuit, and in relating the characteristic of theexpandorline impedance to the expander-line control cirtrolling saidimpedance by the control circuit,

and in relating the characteristic of the impedance to thecharacteristic of the control circuit to obtain a signal volume beyondsaid impedance which has a xed ratio to the signal volume range beforesaid impedance.

V20. In combination, a linerl means connected to the signal line, acontroll cir-A cuit connected to the signal line beyond said impedancemeans to be governedv according to the signals beyond the impedancemeans for control"-l ling the impedance means, means formrelating thecharacteristic of the impedance means with respect to the characteristicof the control cir cuit to obtain a signal volume range beyond saidimpedance means which has a fixed ratio with respect to the signalvolume range before said impedance means, an expandor line havingv sig'-nals supplied thereto that have the same volume range as the outputbeyond the rst mentioned impedance means, a second impedance meansconnected to the expander line, a second control circuit connected tothe expandor line before said second impedance means and having the samecharacteristic as the rst mentioned control circuit for controlling saidsecond impedance means, and means for relating the characteristic ofSecond impedance means to the characteristic or the sec-' ond controlcircuit to obtain an output beyond the second impedance means havingihesame volume range as the input to the rst mentioned impedance means. t

STEPHEN DOBA, JR.

